Digital terrestrial broadcast systems typically transmit OFDM (Orthogonal Frequency Division Multiplex) signals in a single frequency network (SFN). In certain scenarios, however, signals arriving from several (at least two) transmission apparatuses, interfere destructively and the complete signal is canceled (over almost all subcarriers). This behavior was even measured in different field trials, where there are spots in the landscape, which are permanently in deep fade. When the different transmission apparatuses have slightly different frequencies, then these spots move with time, yielding bad reception conditions in various places.
The application of MIMO (Multiple Input Multiple Output) is also an option for increased payload bit-rates and robustness in the currently developed DVB-NGH (Digital Video Broadcasting-Next Generation Handheld) standard. However, the application of MIMO in a broadcasting environment has certain drawbacks. Firstly, it is commonly known that MIMO does not offer high gain for low signal to noise ratios (SNR) if no feedback from the receiver to the transmitter is available, which is most likely the case for DVB-NGH. Secondly, compared to SISO (Single Input Single Output), or SIMO (Single Input Multiple Output) (i.e. equivalent to the already deployed reception diversity), the application of MIMO requires additional pilots for channel estimation. Especially for larger networks, which typically yield many echoes in the corresponding impulse responses, even SISO transmission requires a high pilot overhead to estimate the highly frequency selective channel. This pilot overhead may consume the complete gain offered by MIMO, if the pilot density has to be doubled. Furthermore, the requirement to estimate additional channels leads to additional noise in the estimated channel coefficients.
For instance, the efficiency of the MISO (Multiple Input Single Output) scheme for DVB-T2 (which also uses MIMO pilots) has shown a significant additional degradation due to the channel estimation, which is able to reach up to 0.5 dB additional degradation compared to the SISO case. Hence, it is highly desirable for a multi-carrier transmission system to obtain the MIMO gain in terms of additional diversity, but without having the drawback of the MIMO channel estimation.